In general, (meth)acrylic acid or (meth)acrolein is continuously produced in such a manner that, by use of a multi-tubular reactor having a plurality of reaction tubes in each of which a catalyst is packed, propylene, propane, isobutylene or (meth)acrolein that is a substance to be oxidized is catalytically oxidized with molecular oxygen or molecular oxygen-containing gas in a gas phase in the presence of a composite oxide catalyst.
In the reaction tubes, a temperature distribution is generated in a direction of a flow (in a tube axis direction) of a process gas consisting of a substance to be oxidized and molecular oxygen or a molecular oxygen-containing gas. In general, a temperature peak exists on an upstream side.
FIG. 3 shows a change in a temperature distribution in the tube axis direction resulting from a continuous operation. As an operation is continued, the activity of a catalyst deteriorates and particularly a decrease in the activity of the catalyst is large in the upstream side. Accordingly, a reaction velocity in the region declines to result in a decrease in a heat release value owing to a reaction. Concerning the flow of the gas on a more downstream side than there, as a reaction amount on the upstream side decreases, since a concentration of a raw material that is supplied becomes higher, a reaction amount increases and a heat release value increases. However, a total reaction amount over an entire reaction tube decreases. At this time, a position of a temperature peak in a tube axis direction (hereinafter, referred to as a peak position) moves toward a downstream side and a peak temperature declines (a temperature distribution varies from a to b). To the situation, so far an operation is carried out in such a manner that a temperature of a heat medium (reaction temperature) is raised to bring the peak position back to the upstream side and to heighten a peak temperature, thereby improving the catalyst activity to maintain a yield (the temperature distribution changes from b to c′).